The present invention relates generally to antenna feedlines, and more pa cularly to antenna feedlines for simultaneous modal impedance matching of a multiple mooe. N-fold symmetric or N-fold polygonal antenna.
Conventional feedline technology employs standard transmission line components such as coaxial cables to feed each arm of a multiple-arm antenna. If the transmission lines are isolated from one another (i.e. decoupled), as is typical with conventional antenna feedlines, simultaneous matching of multiple modes cannot be achieved since the coaxial cables remain at a fixed characteristic impedance for all modes. Thus, the feedline and the antenna can only be ideally matched in one of the operating modes. As a result of this limitation, conventional feedline technology is typically designed to have an impedance equal to the average of the antenna operating modal impedances which results in a mismatch loss for each mode and a corresponding power loss in the antenna system.
Accordingly, the present invention provides an antenna feedline for use in feeding signals to or from an antenna feedpoint of a multiple-arm antenna that achieves improved operating efficiencies. Improved operating efficiencies are achieved because the antenna feedline of the present invention is capable of matching the impedance at the connection port(s) of a device connected to the antenna by the feedline such as, for example, a beamformer, an amplifier, a mixer, or an upconverter/downconverter, as well as simultaneously matching all operating modal impedances of the antenna at the antenna feedpoint, within acceptable tolerances. The matched mode antenna feedline of the present invention achieves this by smoothly transitioning its separate transmission lines (e.g., coaxial cables, microstrips, striplines) from a decoupled state at input ends thereof connectable to the device to a highly coupled state at output ends thereof connectable to the antenna feedpoint. The high coupling at the ends of the multi-transmission line connected to the antenna feedpoint results in modal dependent impedances very similar to the modal input impedances of the highly coupled antenna arms at the antenna feedpoint. It will be appreciated, that although the terms xe2x80x9cinput endxe2x80x9d and xe2x80x9coutput endxe2x80x9d are used herein, the xe2x80x9cinput endxe2x80x9d may be outputting a signal to the device that is received at the xe2x80x9coutput endxe2x80x9d from the antenna or receiving a signal from the device to be output at the xe2x80x9coutput endxe2x80x9d to the antenna depending upon whether the antenna is being used to transmit or receive signals.
According to one aspect of the present invention, a simultaneously matched mode antenna feedline includes a plurality of coaxial cable transmission lines. Each coaxial cable extends between an input end thereof and an output end. The coaxial cables are decoupled from one another proximal to the input ends thereof and coupled with one another proximal to the output ends thereof. In this regard, the matched mode antenna feedline also includes a transition section comprising a section of each said coaxial cable between the input and output ends. In the transition section, the outer conductor of each coaxial cable (and also, if desired, the dielectric layer separating the outer conductor from the inner conductor of the coaxial cable) is removed from the coaxial cable in a tapered manner proceeding from proximal to the input end of the cable towards the output end of the cable to smoothly transition the coaxial cables from a decoupled state proximal to the input ends of the cables to a coupled state proximal to the output ends of the cables. The transition section may be configured to provide for a specified rate of increase in coupling between the coaxial cables proceeding from the input ends toward the output ends of the coaxial cables. In order to provide for a smooth transition with little reflection, the transition section preferably has an electrical length equal to or exceeding one quarter of the wavelength of a lowest frequency signal to be fed via the coaxial cables to or from the antenna feedpoint. Shorter transition sections can be used, however, degraded performance in the form of higher mismatch losses at the lower operating frequencies may occur.
The coaxial cable transmission lines may be arranged in a circular cluster. In this regard, it is desirable to keep the diameter of the circular cluster electrically small (e.g., less than about one-tenth of the wavelength of the highest operating frequency) in order to reduce feedline radiation and minimize interaction with the radiating antenna. To further isolate the coaxial cables from radiation radiating from the antenna elements, the transition section may be disposed within an external shield. However, the shield must be located far enough from the feedline to prevent substantial coupling between the shield and the conductors which would interfere with the simultaneous mode matching capability of the feedline.
In the transition section, the outer conductor (and dielectric layer, if desired) of each coaxial cable transmission line may be removed in a linear tapered manner. In this regard, the outer conductor (and dielectric layer) of each coaxial cable may, for example, be cut along a plane intersecting the coaxial cable at an acute angle measured from the input end of the coaxial cable transmission line. The portion of the outer conductor (and dielectric layer) on the side of the plane facing the output end of the coaxial cable transmission line is removed from the inner conductor. The outer conductor (and dielectric layer) of each coaxial cable transmission line may also be removed in a non-linear tapered manner. In this regard, the outer conductor (and dielectric layer) may, for example, be cut along the intersection of a parabolic surface with such coaxial cable and removed from the inner conductor on the side of the parabolic surface facing the output end of the coaxial cable transmission line. It will be appreciated that the outer conductor (and dielectric layer) may be removed in many other different linear and non-linear tapered manners.
According to another aspect of the present invention, a simultaneously matched mode antenna feedline includes a tapered common member and a plurality of coaxial cables. The tapered common member may be comprised of an electrically conductive material such as, for example, aluminum, copper, brass, gold, silver, or alloys thereof. Each coaxial cable extends between an input end thereof and an output end thereof. The input ends of the coaxial cable are decoupled from one another and the output ends of the coaxial cables are coupled with one another. Between the input ends and the output ends, there is a transition section where the coaxial cables are arranged in a circular cluster around the tapered common member and are smoothly transitioned from being decoupled proximal to the input ends thereof to being coupled with one another proximal to the output ends thereof. In this regard, the transition section is provided by removing the outer conductor (and, if desired, also the dielectric layer) of each coaxial cable in a tapered manner proceeding from proximal to the input ends thereof towards the output ends thereof. It will be appreciated that the tapered common member and the transition section of the coaxial cables may be cooperatively tapered in a linear or a non-linear manner.
According to a further aspect of the present invention, a simultaneously matched mode antenna feedline includes a substrate configured in a shape at least partially surrounding a volume. In this regard, the substrate may, for example, be configured in one of a cylindrical shape, a conical shape, and a multiple sided tubular shape (e.g., a square tube, a rectangular tube, a hexagonal tube, or many other polygonal tubular shapes). A plurality of electrically conductive strips are provided on the substrate (e.g., microstrips or striplines). Each strip is oriented longitudinally on the substrate and extends substantially parallel with the other strips between an input end of the strip and an output end of the strip. A single ground plane (microstrips) or two ground planes (striplines) are also provided on the substrate. The ground plane(s) is configured to transition the strips from a decoupled state proximal to the input ends of the strips to a coupled state proximal to the output ends of the strips. In this regard, the ground plane(s) may comprise a plurality of tapered areas, with each tapered area being associated with a separate one of the strips. Each tapered area is wider proximal to the input end of its associated strip and is tapered to a point proximal to the output end of its associated strip. The tapered areas may be tapered in a linear or a non-linear manner proceeding from proximal to the input ends of the strips towards the output ends of the strips. The tapered areas may be or may not be interconnected with one another proximal to the input ends of the strips.
According to one more aspect of the present invention, a simultaneously matched mode antenna feedline includes a substrate configured in a shape at least partially surrounding a volume. In this regard, the substrate may, for example, be configured in one of a cylindrical shape, a conical shape, and a multiple sided tubular shape (e.g., a square tube, a rectangular tube, a hexagonal tube, or many other polygonal tubular shapes). A plurality of electrically conductive strips are provided on the substrate (e.g., microstrips or striplines). Each strip is oriented longitudinally on the substrate and extends substantially parallel with the other strips between an input end of the strip and an output end of the strip. One or more grounding members is disposed within the volume. The grounding member(s) is shaped to transition the strips from a decoupled state proximal to the input ends of the strips to a coupled state proximal to the output ends of the strips. In this regard, the grounding member may be conically shaped.
According to yet another aspect of the present invention, a simultaneously matched mode antenna feedline includes a plurality of coaxial cables. Each coaxial cable extends between an input end thereof and an output end. The coaxial cables are decoupled from one another proximal to the input ends thereof and coupled with one another proximal to the output ends thereof. In this regard, the matched mode antenna feedline also includes a transition section comprising a conically shaped electrically conductive member and a section of each coaxial cable between the input and output ends thereof wherein the outer conductor (and also the dielectric layer, if desired) of each coaxial cable is removed therefrom. The conically shaped electrically conductive member may be linearly or non-linearly tapered from its base to its apex. The sections of the coaxial cables from which the outer conductor (and dielectric layer) have been removed are inserted in a corresponding plurality of holes extending from the base of the conically shaped electrically conductive member therethrough parallel with a central axis of the conically shaped electrically conductive member. In this regard, the holes may be arranged in a circular cluster or a polygonal cluster around the central axis of the conically shaped electrically conductive member.
In summary, a simultaneous mode matching feedline in accordance with the present invention includes four general characteristics. The first characteristic of the feedline is a smooth transition between isolated transmission lines at the input end and coupled lines (multi-transmission line) at the output end. The second characteristic is the plurality of transmission line configurations that the feedline can be comprised of including coaxial, stripline, microstrip, coplanar strips, and coplanar waveguides and which can be configured in cylindrical, conical or multiple-sided tubular shapes (e.g., rectangular, hexagonal). The third characteristic of the simultaneous mode matching feedline is that the transition section can be tapered in a plurality of linear and non-linear configurations to achieve a smooth transition from isolated transmission lines at the input end to a coupled feedline at the output end. The fourth characteristic of the feedline is that it can be comprised of any number of isolated transmission lines at the input end which transition to the same number of coupled lines at the output end, that number being equal to the number of coupled antenna elements that the lines are connected to at the feedpoint.
These and other aspects and advantages of the present invention will be apparent upon review of the following Detailed Description when taken in conjunction with the accompanying figures.